diff --git a/README.md b/README.md
index 543a068..131d7a1 100644
--- a/README.md
+++ b/README.md
@@ -41,7 +41,7 @@ namespace. Ideas:
 For the first version of the keys, we will use 3 bytes encoded as base64url,
 as described in [RFC4648, par. 5]. To allow for truncate-resistant upgrading
 of the URL key format, the first bit is set to `0`. That means the first base64
-character will always be in `[A-Za-f]`.
+character will always be in `/[A-Za-f]/`.
 
 This domain contains `8388608` values. If the service is somewhat used, we will
 get collisions early when we generate those keys at random. Instead, we will